Volume control in receiver sets



0`t.15,5194o. H SCH ETAL 2,218,095

`VOLUME: CONTROL IN RECEIVER SETS FiledOct. 27. 1937 (54am/4,90 4er/cw) 4 HELMUT PxTscH l FICE STATES VOLUME CONTROL 1N RECEIVER SETS Helmut Pitsch, Berlin-Steglitz, and Otto Txen, Berlin, Germany, assignors to 4'Ielefunken Gesellschaft fr Drahtlose 'Eelegraphie m. b. H., Berlin, Germany, a corporation of Germany Application october 27', its?, serial No. 171,204

In Germany October 28, 1936 UNITE 3 Claims. (Cl. 179-171) In order to diminish non-linear harmonic dis- Vvalue to suit a driving range up to incipient grid tortion, in the presence of low volume, it has been current ilow, for a given working point. The suggested to increase the external resistance of characteristic is chosen so steep that theworking the power tube of amplifying apparatus. Acpoint will divide a given grid voltage range into l i cording to the present invention, the external two parts being as far as feasible equal` 5 resistance serves solely for volume control with With growth of external resistancethe working the simultaneous use of several other objects of characteristic A Hbecomes less-steep (A1). Inasthe invention. f i much as the triangles indicated in the drawing The aim of the invention is to create volume are va measure and criterion for theA delivered l control means adapted to minimize distortion by power, it will be noted that with increase'of exl0 the power tube regardless of position or adjustternal resistance, in spite of the same grid alterment. According to previous notions it has been hating potential, the volume willl decrease. The held that if the volume or signal strength is conprerequisite condition, namely, that the external trolled and regulated at the output end of a reisistance should then differ more widely` from power tube the distortions are more pronounced the internal resistance is automatically fulfilled, I1li than when the same action is produced at the if the above rule of dimensioning the external input end of the power tube. However, as demresistance is satisfied, for modern radio tubes. onstrated by the invention even a reduction of Figure 2 which shows the dependence of the y the non-linear harmonic distortion is feasible if power upon the non-linear harmonic distortion 2o certain features and steps as here disclosed are for increasing outer Vresistance `(characteristics ..0

simultaneously used. 1 3) makes it clear that the non-linear har- The invention consists in varying the external monic distortion decreases with increasesin the.

resistance of a triode power tube being of suitable external resistance. l proportions so as to insure full driving range up yThe markedA rise Vof the characteristic when to a point where grid current starts to flow, in getting into the hatched portions corresponds to such a wai7 that the volume decreases with inthe startA of grid current flow: If the external crease of the said resistance, and that at the resistance is not altered, one would get, say,from same time the highestoccurring grid alternating point a to point b as. the signal voltage (grid alpotential impressed upon the power tube is only ternating voltage) decreases. But if the volume so high, Or the power tube of such dimensions, is `reduced by raising the value of theexternal 30 that an appreciable overloading owing to nreisistance, one might get, say, to point c from cipient grid current flow will not happen when point a, while yet the same volume (power) is the apparatus is adjusted to maximum sound obtained asat b, though with a non-linear disvolume, means being moreover provided to the tortion.

end of stabilizing the mean grid alternating The risk of overload in'this instance is not 35 (signal) voltage (for instance, gain regulation or greater than for the maximum` sound volume, phonograph disks). with a resistance corresponding to graph 1, pro- In the drawing, Figs. l Vand 2 are curved sheets' videdthat the highest occurring signal voltage is used in explaining the invention and Fig. 3 is only so high, or the power tube is so amply 1pro l 4c a schematic diagram of a radio receiver incorportioned, that, even when' the set is adjustedto 40 porating the features of the present invention. maximum volume, no overload due to incipient yBy reference to Fig. l of the appended drawgrid current iiow willoccur. rThis can be seen ing it shall rst be shown that in the presence of also from Fig. l where the characteristic 0.oorre-4 an. increase of the external resistance (say, by spends roughly tothe beginning of'grid current.

an increase in the transformation ratio of the Another presupposition, according toja further 45 output transformer for a dynamic loudspeaker), feature and object of the invention as herein'-l the delivered power or sound `volume will debefore mentioned is that, for instance, by satiscrease in spite of unvaried grid alternating voltfactory volume control care has been taken so age. In the drawing is graphically illustrated that the mean signal or grid alternating voltage the well-known family of curves showing the will be constant so that the manually operated 50 dependence of the plate current Ia upon the plate volume control, apart from the equalization of potentialUa of a triode (single-grid) type of tube. differences in modulation percentage of the vari- These various graphs apply to different grid poous transmitter stations, serves solely for the tentials. Working characteristic A corresponds adjustment of the set to the desired volume.

to the external resistance chosen of optimum v Application of this invention to pentodes is not 55 practicable for the reason that, inside a relatively large range of size of the external resistance, the power varies but little, but the non-linear harmonic distortion very markedly.

Where the present invention is carried into practice, the input tubes must be proportioned so generously that the distortions liable to arise in practice will be moderate even for maximum drive ranges.

It is to be noted that with an increase of the external resistance of the power tube, by raising the transformation ratio of the output transformer, say, by means of taps, the high-pitched notes will be somewhat weakened, though this could be compensated by the aid of equalizer means.

Another manner adapted to regulate the outer resistance as shown in Fig. 2 is accomplished by connecting a resistance Win series with the loudspeaker coil. In this way the said drawback is obviated seeing that, upon rise of the loudspeaker resistance, for instance, for high notes, a larger shareof the power applied to the series resistance W and the loudspeaker will go to the loudspeaker than is true for the low-pitched notes. On the other hand, by the use of a series resistance, the efficiency is impaired so that the external resistance, to obtain a certain volume reduction may rise somewhat less. 'I'he improvement of conditions regarding the non-linear harmonic distortion will then be correspondingly smaller, yet, it will nevertheless be present. By theway, the two ways and means adapted to volume control as just mentioned could also be used simultaneously.

The series resistance should preferably not be included in the output circuit of the power tube traversed by D. C. in order that the operating potentials of the power tube may not be unfavorably'affected. -It may also be noted that parallel connection of an ohmic resistance to the loudspeaker will not attain the aim and end of the invention, quite on the contrary.

In the exemplified embodiment Fig. 3, the portion of the receiver E located below the power tube is provided with fading compensator (avc) means with forward and backward action. The RF energy utilized for such control action, as willbe seen, is rectified in rectifier G, the resultant avc potential being impressed upon a lower and a higher stage. The 'amplitude fed to the power tube may once forever be adjusted to an optimum value'by choosing corresponding dimensions for the coupling means. q The invention will be found useful also in AF ampliiiers comprising means for gain regulation. In case of amplification for the playing of phonograph records, a limitation is imposed uponfthe amplitude alone on the ground that the sound volume produced by a record is unable to surpass certain values conditioned by a given groove or track width and a given gauge of the needle.

It is claimed:

1. In amplifying apparatus, a power tube having a cathode, a grid and an anode, an input circuit for said tube connected between the grid and cathode thereof, an output circuit connected between the anode and the cathode of the tube, a load, means for coupling the tube output circuit and the load, said means being arranged to substantially match the impedance of the load to the impedance of the tube for maximum power transfer, a driving circuit for said tube coupled to the input circuit thereof said drivingY circuit including means for stabilizing the signal energy impressed upon the input circuit of the tube and to limit its maximum amplitude to a value such that the highest occurring alternating current potential impressed upon the grid` will not cause grid current to flow at maximum power transfer between the tube output circuit and the load, and supplementary means for increasing the effective external plate circuit alternating current impedance of the tube to thereby reduce both the non-linear distortion of the tube and the power transferred to said load.

2. In signalling apparatus, a power amplifier tube including a grid circuit and a plate circuit, 'a load, coupling between the plate circuit and Athe load including matching means adapted to reflect. into the tube circuit an impedance at least as high as the output impedance of said tube, a driving circuit for the tube coupled to the grid circuit thereof, said driving circuit including means for stabilizing the signal energy impressed upon the grid circuit of the power tube and limiting its maximum amplitude to a value such that the highest occurring alternating potential impressed upon the grid will not cause grid current to flow at maximum power transfer between the plate circuit and the load, and a resistance in series with said load for increasing the effective external output circuit alternating current impedance of the tube to thereby provide for the simultaneous reduction of both the non-linear distortion of the tube and the power delivered to the load.

3. In a signalling circuit, a power amplifier tube provided-with a plate circuit including the primary winding of an output transformer, and a gridcircuit, a driving circuit for said tube coupled to said grid circuit and including means for stabilizing thesignal energy impressed upon the grid circuit and limiting its maximum amplitude to a value such that the highest occurring alternating current impressed upon the grid circuit will not cause grid current to iiow at maximum power transfer between the plate circuit and the load and said load in series connected across the secondary 'winding of said output transformer, said transformer comprising a matching means arranged to reiiect into the tube circuit an impedance at least as high as the output impedanceV of said tube, said variable resistor providing'means for reducing both the non-linear distortion of the tube and the power delivered by the tube to the load without varying any direct current operating potential applied to the tube.

HELMUT PITSCH.

o'rro TXEN. 

